text: With few exceptions (e.g., deoxyribose), monosaccharides have this chemical formula: (CHO), where conventionally x ≥ 3.  Monosaccharides can be classified by the number x of carbon atoms they contain: triose (3), tetrose (4), pentose (5), hexose (6), heptose (7), and so on.
  Glucose, used as an energy source and for the synthesis of starch, glycogen and cellulose, is a hexose. Ribose and deoxyribose (in RNA and DNA, respectively) are pentose sugars. Examples of heptoses include the ketoses, mannoheptulose and sedoheptulose. Monosaccharides with eight or more carbons are rarely observed as they are quite unstable. In aqueous solutions monosaccharides exist as rings if they have more than four carbons.